Smallpox was believed afferent before the Sui Dynasty. Some relatively accurate descriptions about smallpox were found in the Sui and Tang Dynasties. Smallpox was recognized as a severe infectious disease that caused by pathogenic heat by the observation of its wapoliex symptoms and appearance. was caused by the heat accumulation in the Zang-Fu organs in the early period of the North Song Dynasty. After that, it was believed caused by the fetus ingesting the filthy liquid from the mother as the prototype of the fatal poison theory. that ancient Chinese physicians were able to distinguish the differences among chickenpox, measles and smallpox at that time. The exogenous theory of fetal poison became the mainstream in the Ming and Qing dynasties. Smallpox, therefore, was confirmed not to be re-infected after once infection, and was specifically described with the symptoms and the precise prognosis. It can be seen that the understanding of smallpox by ancient Chinese physicians developed with time, and these understandings opened a way for the invention of the human pox vaccination